<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <h1>跨域请求</h1>
    <button>3000服务器向4000服务发送请求</button>
    <script>
     let btn = document.querySelector("button");
     btn.onclick = function () {
         let xhr =new XMLHttpRequest();
         xhr.open('get',"http://localhost:4000/getdata")
         xhr.send();
         xhr.onload= function () {
             if (xhr.status>=200&&xhr.status<300) {
                 console.log(xhr.responseText);
             }
         }
     }
    </script>
</body>
</html>